Multi-range hydromechanical transmissions are well known. Ranges are changed by sequentially disengaging and engaging a number of clutches. The engagement typically occurs under synchronous or near synchronous conditions. That is to say that ranges can be changed without introducing discontinuities into the continuously variable nature of the transmission.
U.S. Pat. No. 5,080,637 (“637”) teaches a hydromechanical transmission. The hydromechanical transmission receives split power inputs from an engine; one split input drives a hydrostatic transmission and the other split input drives a mechanical power input. Power combines in the planetary gear set and a series of multi-disc clutches and brakes provide shifting between ranges.
The '637 hydromechanical transmission discussed above shifts through multiple transmission ranges using at least four clutches and two brake assemblies. While multi-disc clutches allow very flexible engagement strategies, they are deficient in several respects. In particular, multi-disc clutches are costly, complex, bulky, and generate losses when not engaged. Market pressures are increasing for a continuously variable transmission with low losses, low complexity, and low cost.
Dog clutches are an alternative to multi-disc clutches. U.S. Pat. No. 3,916,714 to Sisson et al. teaches a sleeved shift-connecting member that is positionable between one of two positions. Selected gear assemblies, in order to provide the desired operation range, must be intermeshed through a dog clutch, which, in turn, necessitates that the gears include negligible rotational speed to carry out the shift. They are economical, compact, and have low losses. However, dog clutches are extremely sensitive to shift conditions, i.e., they must be perfectly synchronized to allow engagement, and they must be unloaded to allow disengagement.
The disclosed transmission is directed to overcoming one or more of the problems set forth above.